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Difficulty: MediumMountain Driving and Steep Downgrades

A commercial driver is preparing to safely negotiate a steep mountain downgrade. Arrange the following driver actions in the correct chronological order from pre-descent preparation through the completion of a proper snub braking cycle.

  1. 1Shift the transmission into a low gear that provides sufficient engine braking before beginning the descent.
  2. 2Allow gravity and momentum to accelerate the vehicle until it reaches the predetermined safe speed.
  3. 3Apply the service brakes firmly enough to feel a definite slowdown.
  4. 4Maintain steady brake application for approximately 33 seconds until vehicle speed drops to 5 mph5\text{ mph} below the safe speed.
  5. 5Release the service brakes completely to allow the brake drums to cool while vehicle speed slowly builds back up.

Answer

Shift into low gear before descent → Allow vehicle to reach safe speed → Apply brakes firmly to feel a definite slowdown → Reduce speed to 5 mph5\text{ mph} below safe speed → Release brakes completely to cool.
The established FMCSA procedure requires shifting into the proper low gear prior to cresting the grade. Once descending, the vehicle is allowed to reach its designated safe speed, at which point the driver applies firm brake pressure to reduce speed by 5 mph5\text{ mph} (taking about 33 seconds) before completely releasing the pedal to allow brake cooling. Repeating this snub cycle maintains control without overheating the foundation brakes.

Step-by-Step Solution

1
Select and engage the proper low gear before cresting the summit or starting down the grade.
The engine provides maximum retarding force without risk of missing a gear on the slope.
Modern commercial transmissions cannot safely be downshifted once descending steep grades without risking transmission lockup or missing gears into neutral.
2
Monitor the speedometer as the vehicle travels downgrade until it reaches the designated safe speed.
The vehicle establishes the upper speed threshold for the snub braking cycle.
Safe speed is calculated based on total weight, grade steepness, road conditions, and length of grade.
3
Apply the service brakes with moderate, steady pressure.
The driver feels a definite deceleration.
Firm application ensures all brakes engage evenly and slows the vehicle efficiently.
4
Hold the brake pressure until speed decreases to 5 mph5\text{ mph} below the safe speed.
Vehicle velocity drops within approximately 33 seconds.
Dropping 5 mph5\text{ mph} below the target speed creates an adequate buffer for cooling during the release interval.
5
Release the brake pedal completely until vehicle speed climbs back to the safe speed.
Brake linings and drums dissipate heat during the unbraked interval.
Full release prevents continuous heat buildup that causes dangerous thermal brake fade and expansion.

Key Concept

Proper downgrade speed management relies on pre-descent low gear selection combined with cyclic snub braking (braking to 5 mph5\text{ mph} below safe speed and fully releasing) to avoid brake fade.
Estimated Time:1m 15s
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Topics

Mountain Driving and Steep Downgrades

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